#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Try the calculation of char pointers cf. array subscription
// I did not figure out the char pointer can be implicitly converted to integer number
//

int main()
{
    char str[] = "The quick brown fox jumps over the lazy dog!";
    char *line;
    char *p, *q, *l, *m;

    line = (char *) malloc(sizeof(str));    //Line 15 allocate memory 

    strcpy(line, str);            //Line 17 copy char array to char pointer 

    p = strstr(line, "fox");    // Find the position of "fox" in line
    q = strstr(line, "dog");    // Find the position of "dog" in line

    l = strstr(line, "quick");    // Find the position of "quick" in line
    m = strstr(line, "lazy");   // Find the position of "lazy" in line

    printf("Postion subtraction q(%p) - p(%p) = %d\n", p, q, q - p);        //Line 25
    printf("Postion subtraction m(%p) - l(%p) = %d\n", m, l, m - l);        //Line 26

    printf("Char value subtractn: o(%c) - q(%c) = %d\n", str[12], str[4], str[12] - str[4]);
    printf("Char Value subtractn: q(%c) - e(%c) = %d\n", str[4], str[2], str[4] - str[2]);
    printf("Position subtraction: &q - &e = %d\n", &str[4] - &str[2]);        //Line 30

    return 0;
}

$ gcc -Wall ptr_substraction001b.c
ptr_substraction001b.c: In function ‘main’:
ptr_substraction001b.c:25:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘long int’ [-Wformat]
ptr_substraction001b.c:26:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘long int’ [-Wformat]
ptr_substraction001b.c:30:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long int’ [-Wformat]

$ ./a.out
Postion subtraction q(0x987020) - p(0x987038) = 24
Postion subtraction m(0x987033) - l(0x987014) = 31
Char value subtractn: o(o) - q(q) = -2
Char Value subtractn: q(q) - e(e) = 12
Position subtraction: &q - &e = 2